1. Field of the Invention
The present invention relates to an improved motorcycle frame.
2. Description of Background Art
In a motorcycle, a structure is adopted where front forks and a steering handlebar are attached to a head pipe via a handle rotating shaft, and a front wheel is attached to the front forks so that the motorcycle can be steered, with the head pipe functioning as a central pivot point. Because a front frame extends rearwardly from the head pipe, or a down tube extends diagonally rearwardly from the head pipe, a frame structure where the end of the front frame and the down tube are formed integrally with the head pipe is generally adopted.
For example, FIG. 2 of Japanese Patent Laid-open No. Hei. 6-107261, entitled Motorcycle Frame Structure, shows a head pipe 5 and a front frame 4, while a head pipe section 5A and a frame section 14Axe2x80x2 are shown in FIG. 4.
The head pipe section 5A shown in FIG. 4 of the above mentioned publication is described as a xe2x80x9cforged componentxe2x80x9d with a frame section 14Axe2x80x2 integrally welded to the forged component as discussed in paragraph 0018, lines 4 to 6 of Japanese Patent Laid-open Hei. 6-107261.
However, it is difficult to make the forged component strong compared to a rolled member or an extruded member, and it must be made thick in order to compensate for this. If the forged member is made thick, the weight of the frame is also increased. Also, since the head pipe section 5A is a forged component and welding is more complicated for a forged component compared to a rolled member or an extruded member, the number of welding steps is increased. Accordingly, if the forged head pipe is adopted there is a resulting increase in weight and manufacturing cost.
Also, as described in paragraph 0011, line 7 of the above described publication, the head pipe 5 and front frame 4 of FIG. 2 mentioned above are integrally formed by casting or forging. However, as shown in FIG. 3 of the same publication, a rear edge 17A of the front frame 4 has a cross section in the shape of a three-sided square, and is what is known as undercut, and is difficult to forge, which means that forging becomes 2-stage (the undercut section is formed in a later stage) or multiple stage forging having three or more stages, and forging cost is increased, making actual production unfeasible.
When the head pipe and the front frame are integrally formed by casting, their strength is reduced compared to a head pipe and a front frame made of a rolled member or extruded member, and it must be made very thick to compensate for this. Increased thickness makes the frame heavier.
There are problems associated with cast and forged parts of the related technology, but generally speaking it can be said that forged parts are much stronger than cast parts.
An object of the present invention is therefore to provide a motorcycle frame structure, adopting a head pipe manufactured by forging, that enables significant reduction in forging costs, and enables reduction in welding costs when welding hollow frames such as a front frame and down tube etc.
In order to achieve the above described objects, the present invention is directed to a motorcycle frame structure, the motorcycle frame joining together a head pipe and hollow frame extending backwards from the head pipe by welding, the head pipe being made into a pipe by opening a longitudinal hole in a forged component forged using left and right dies, and an upper tongue and a lower tongue gripping a tip of the hollow frame being integrally formed at the time of forging.
An upper tongue and a lower tongue gripping a tip of a hollow frame are integrally formed in the forged component, and the hollow frame is inserted between the upper and lower tongues and welded.
Since only the upper and lower tongues are integrally formed with the forged component, there is no need to include undercut processing in the forging process. Accordingly, it is possible to reduce the manufacturing cost.
In addition, because the hollow frame is inserted between the upper and lower tongues, positioning of the tip of the hollow frame is straightforward, initial set-up before welding is simplified, and holding the hollow frame is easy, which means that the welding operation is simplified. If the hollow frame is welded to the upper and lower tongues, it is possible to ensure sufficient weld length. Accordingly, it is possible to reduce the welding costs.
The present invention is directed to a motorcycle frame structure, the motorcycle frame joining together a head pipe and hollow frame extending backwards from the head pipe by welding, the head pipe being made into a pipe by opening a longitudinal hole in a forged component forged using left and right dies, and a flange insertion seat, comprising a vertical wall shaped web section extending backwards along a center line of the vehicle, upper and lower flange sections extending to the left and right from the web section, and a longitudinal frame section, being integrally formed at the time of forging for insertion of left and right hollow frames.
At the time of forging, a wall-like web section, upper and lower flange sections extending laterally from this web section and a vertical flange section are also integrally formed by left and right divided molds. The web section can be formed between left and right molds, while the upper and lower flange sections and the vertical flange section can be formed swelling out by forging, which means that the flow of metal at the time of forging becomes smooth and it is possible to obtain a high strength forged component.
A frame insertion seat is formed in the forged component, and the hollow frame is inserted into the frame insertion seat and welded.
Since the frame insertion seat is only integrally formed with the forged component, there is no need to include an undercut in the forging molds. This makes it possible to reduce the forging cost.
In addition, because the hollow frame is inserted into the frame insertion seat, set-up before welding is simplified. If the hollow frame is welded into the frame insertion seat, it is possible to ensure a sufficient weld length, which makes it possible to reduce the welding costs.
The present invention is directed to the head pipe having small left and right windows formed by a punching process in the forged component together with the longitudinal hole, and ends of the left and right hollow frames can be inserted into these small windows prior to welding.
Small windows into which the hollow frame will be inserted are formed together by punching.
Since there is no need for special operations to punch the small windows, it is possible to reduce the cutting costs.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.